Correctly store and pass the jshintrc filename into runner.js

Previously, `s:FindRc` was storing a list of a string containing the contents of a .jshintrc in `s:jshintrc` -- this was never used elsewhere. The argument wasn't being passed into `runner.js`, which accepts a filename anyway, not a string of the contents.

This fixed using a .jshintrc file anywhere in the upward directory path for me, but I could very well be doing it completely wrong, and VIMScript is horrifying so there's probably a better way to write it. :icecream: